1. Field of the Invention
The invention relates to a silicon mesa structure integrated in a glass-on-silicon waveguide for transmission and modulation of electromagnetic radiation.
The invention also relates to a method of manufacturing a silicon mesa structure integrated in a glass-on-silicon waveguide for transmission and modulation of electromagnetic radiation.
2. Description of the Related Art
Glass on silicon is the most promising system of materials which has been developed till now with a view to manufacturing integrated optics. The manufacturing process is inexpensive and compatible with silicon based microelectronics. A large number of low loss passive components may be manufactured on large substrates, good low loss coupling to optical fibres is possible, and low rate (less than 1 MHz) modulation has been demonstrated by the use of the photo-elastic effect. The first active components (amplifiers and lasers) in glass-on-silicon waveguides have been demonstrated, but to provide optical sources and detectors for the communication wavelengths (1.3-1.6 xcexcm), it is necessary to use hybrid integration. Also, modulation at frequencies above 1 GHz has been demonstrated by the use of hybrid integration with semiconductor materials. This is complicated as well as expensive. This has led to experiments with direct integration of polymer material in glass-on-silicon waveguides for use as optical modulators. The use of polymer materials is associated with a reduced service life of the component and thereby a reduced reliability of the modulator.
For the transfer of information through an optical communications system it is required that a property of the light is changed in accordance with the information. Information may thus be transferred to a light wave by changing its intensity, phase, frequency, polarization or direction in analog or digital form. In particular, modulation of the intensity of light at high frequencies (greater than 1 GHz) is of great importance in connection with the input of information into communications systems. Likewise, modulation of the direction is of potentially great importance in optical time division multiplexing and demultiplexing.
Planar optical waveguides for modulation of light are well-known. The modulation is generated e.g. by varying the refractive index or the propagation loss in the waveguide. This may be achieved e.g. by changing the concentration of free charge carriers in a semiconductor material. Thus, silicon may be used as a modulator, the concentration of free charge carriers being controlled through a pn-junction. The development of such semiconductor based optical components has taken place in relative isolation with respect to glass-on-silicon components.
Silicon based modulators are typically manufactured by depositing low-doped epitaxial silicon directly on a high-doped silicon substrate or an oxidized silicon substrate (in a Silicon-on-insulator structure (SOI)). A one-dimensional waveguide (film waveguide) is provided, as low-doped silicon exhibits a higher refractive index than high-doped silicon owing to the difference in concentration of free charge carriers in the materials. A two-dimensional waveguide may be provided by forming a ridge in an epitaxial material and through doping adjacent areas of the core. An electro-optical modulator is provided in this type of waveguide by introducing p-type and n-type doping in the silicon material in the sheath layers of the three-dimensional waveguide. These layers may be introduced in a lateral direction as well as in a vertical direction, so that n-type and p-type materials are present on their respective sides of the core of the waveguide. The doping may be carried out so that a single pn-diode or a number of these (through which unipolar or bipolar transistor structures may be formed) are created. Another method of controlling the number of free charge carriers in the waveguide is by making a metal oxide semiconductor transistor (MOSFET) structure. This structure, too, may be provided longitudinally of or transversely to the waveguide.
It is typical of silicon based waveguides that they exhibit relatively high propagation losses because of the high-doped substrates. The use of SOI substrates reduces this by increasing the distance between the waveguide and the high-doped substrate. The reduce the propagation loss to below 0.1 dB/cm it is necessary to use insulated layers which are at least 2 xcexcm thicker than what is available as standard SOI wafers. For silicon to be of real interest as a medium in an integrated optical modulator, it is necessary that the rapid electrically controllable properties of silicon must be combined with passive low loss integrated waveguides (preferably less than 0.1 dB/cm). It has not been possible to satisfy this requirement with the previously proposed semiconductor based structures.
The object of the present invention is to provide a rapid and reliable electro-optical modulator for glass-on-silicon waveguides. The object of the invention is achieved by introducing a silicon mesa structure into the core or the sheath of a glass-on-silicon waveguide. Hereby, the reliable and rapid electrically controllable modulation of the silicon mesa structure are combined with the low propagation loss of the glass-on-silicon waveguide.
A waveguide may be provided in the silicon mesa structure by making the core of the waveguide of low-doped monocrystalline silicon, while the sheath may be provided by doping parts of the mesa structure and/or making parts such that these preferably consist of silicon dioxide. Here, the structure may advantageously be made of the silicon substrate material. This gives the advantage that the use of expensive epitaxial silicon or SOI substrates can be avoided. To achieve optical wave guidance in a waveguide composed of a silicon mesa structure made of the silicon substrate material, it is necessary to remove excess silicon substrate material below the mesa structure. A further advantage is that the propagation loss in the silicon mesa structure is reduced by removing excess silicon substrate material below it.
By changing the concentration of free charge carriers in the silicon waveguide, it is possible to change the effective refractive index in it. This results in a change in the optical path, which leads to a change in the phase of the light in the waveguide over the given extent. Optical path is here a measure of the time it takes the light of a given wavelength and type to pass through the silicon waveguide.
For the change of the refractive index through variation in the concentration of the free charge carriers in the waveguide, it is expedient if the silicon waveguide is constructed as a diffused pin diode. By applying an electrical signal in the forward direction or reverse direction of the diode, charge carriers may be injected into or be depleted from the diode. To achieve fast modulation, the modulator, in a preferred embodiment, will be based on the depletion of charge carriers. This is achieved by biasing the diode in the reverse direction and modulating this voltage.
It is an advantage if the silicon modulator is constructed such that the glass-on-silicon waveguide is recessed in the silicon substrate. This ensures that the optical axes of the glass-on-silicon waveguide and the silicon waveguide coincide. This enables coupling of light from the glass-on-silicon waveguide to the silicon waveguide (and vice versa). It is a further advantage if the glass-on-silicon waveguide is constructed such that the glass-on-silicon waveguide is recessed in the silicon substrate and the upper sheath glass is limited to cover just the recessed part of the waveguide. This structure reduces the double refraction known in glass on silicon, which occurs because of the difference in thermal expansion of the silicon substrate and the glass structure. The advantage is not limited to a glass-on-silicon waveguide with a silicon mesa structure inserted into the core and/or the sheath of a glass-on-silicon waveguide. The effect of removing excess glass from the surface of the silicon substrate applies generally to all types of glass-on-silicon waveguides. Here, a clear reduction in the double refraction known for glass on silicon can generally be achieved.
The invention thus combines the electro-optical properties of silicon, which give rise to a relatively high transmission loss (0.5-1.0 dB/cm), and the low transmission loss (0.02-0.05 dB/cm) of the passive glass-on-silicon waveguide.
It is noted in this connection that it is an advantage to use adiabatic transmission of light power between the glass-on-silicon waveguide and the silicon waveguide (and vice versa). The silicon waveguide exhibits a very small spot size (wave type profile) because of the relatively high refractive index in the core. In contrast to this, glass on silicon exhibits a spot size which is comparable to the spot size in an optical fibre. Thus, a silicon waveguide will exhibit a spot size (1/e2 diameter) which is 1.5-2.5 xcexcm, while an optical fibre typically exhibits a spot size of about 10 xcexcm, both at a wavelength of 1.55 xcexcm. An adiabatic coupling ensures that the spot sizes of the two waveguides are adjusted to each other by a gradual change of the profile from the dominating form in one waveguide to the dominating form in the other waveguide. Thus, the transmission allows passage of light between the two waveguides without loss of optical power. An adiabatic transmission may advantageously be provided, in that the end members of the silicon waveguide section are formed such that these have a width which decreases progressively over a given length toward the glass-on-silicon waveguide.
With a view to using the silicon waveguide for modulation of the phase of the light, it is an advantage that the glass-on-silicon waveguide and the silicon waveguide section just allow transmission of the fundamental wave type. This gives the best interplay between the light and the modulation signal. A monomode waveguide may be produced by dimensioning the size of the intrinsic area of the silicon waveguide section so that it just supports the fundamental wave type. This may e.g. be combined with a ridge of the cross-section of the waveguide and by diffusing n-type or p-type doping in the material on the sides of the step.
The advantage of a diffused profile is that irregularities in the surface are reduced by the diffusion. The use of a diffused profile results in a concentration of both the propagating light wave and the electrical modulator signal in the same part of the waveguide structure. Moreover, a buried step profile will allow a more equal distribution of the change in the size of the intrinsic area.
For an application as an intensity modulator or a wavelength filter, it is expedient that at least one silicon mesa structure is arranged in an interferometric configuration.
When using the silicon waveguide of the invention for directional modulation, it is an advantage if it contains several sections which may be modulated individually. These sections may be arranged in extension of each other as well as at the side of each other, this both in parallel and with a small angle between the individual sections.
Expedient embodiments of the silicon waveguide are defined in the dependent claims in general.
As mentioned, the invention also relates to a method.
This method is characterized by comprising the steps of:
a) applying a mask to the front side of a silicon substrate and forming a mesa structure, following which a first layer of sheath glass is formed,
b) applying a further mask to the front side of the silicon substrate and removing parts of the first layer of sheath glass,
c) forming an optical waveguide consisting of a core and an upper sheath glass on the front side of the silicon substrate,
d) applying a further mask to the front side of the silicon substrate and removing parts of the upper sheath glass and the core, following which a first area in the silicon mesa structure is doped,
e) applying a further mask to the rear side of the silicon substrate and removing excess substrate material below the silicon mesa structure,
f) doping the rear side of the substrate and a second area of the silicon mesa structure,
g) applying a diffusion barrier to the front and rear sides of the silicon substrate,
h) applying a mask to the front side of the silicon substrate, through which mask holes in the diffusion barrier and the metallization to a first area of the silicon mesa structure are formed,
i) applying a mask to the rear side of the substrate, through which mask holes in the diffusion barrier and the metallization to a second area of the silicon mesa structure are formed.
Expedient embodiments of the method of the invention include forming the first layer of sheath class by chemical oxidation and/or forming the diffusion barrier by Si3N4. As a further embodiment, the removal of excess substrate material below the silicon mesa structure may be carried out using an electrochemical etch stop.